UV LED curing has been the subject of significant development over the past few years because of the advantages of low temperature operation and extremely long life in comparison with conventional medium pressure mercury arc curing lamps. UV LED lamps are advantageous because of the inherently small size of LED units and their ability to be easily engineered into commercial printing systems.
US 2002/0149660 (Arthur L Cleary and Joseph A Lahut) entitled “Apparatus and method for setting radiation curable ink” describes the equipment and method for curing inkjet ink using UV LED lamps. U.S. Pat. No. 7,175,712 (Con-Trol-Cure Inc.) entitled “Light emitting apparatus and method for curing inks, coatings and adhesives.” describes staggered rows of UV LED chips arranged in such a way as to allow UV curing of inks and coatings on a moving web. US 2005/0104946 (Con-Trol-Cure Inc.) entitled “Inkjet UV curing” describes the way in which UV LEDs can be mounted and used to cure UV inkjet inks or least partially polymerize them and also the need to use an inert gas environment to improve the cure by suppressing oxygen inhibition. No details of the formulations that are particularly suitable for curing using LED lamps are disclosed in those documents. US 2005/0128274 (Konica Minolta Holdings Inc.) entitled “Inkjet Printer” describes fully integrated inkjet printing units fitted with UV LED curing that avoids problems of heat from conventional curing lamps and is also smaller and cheaper to build. The application also describes UV inks as either free radical or cationic in nature but does not provide details of any of the photoinitiator types necessary to cure such a composition. While Mercury arc lamps typically have a polychromatic emission spectrum, emitting light in all regions of the UV-visible spectrum of from 200 to 450 nm, UV LED lamps typically have only a single emission band at a UV wavelength towards the longer end of the spectrum, for example 365-420 nm, typically about 395 nm. It is widely accepted that short wavelength UV light is responsible particularly for “surface cure” in inks and coatings, whereas long wavelength light has much higher penetration and is responsible for much of the “through curing”. The reduced total UV output associated with UV LED lamps as opposed to typical medium pressure mercury lamps places two significant restrictions on a UV ink formulator, firstly surface cure is more difficult to achieve, and secondly the number of photoinitiators that absorb light in the region of LED emission is very small and makes effective formulating more difficult.
When using UV LED lamps or other monochromatic UV light sources to cure inks and coatings, it is necessary to use photoinitiator systems that are tuned to the wavelength of the light source. WO 2005/111128 (Flint Ink Corporation) entitled “Ink for Excimer curing”, describes photoinitiator formulations suitable for the curing of lithographic printing inks using Excimer lamps such as the “Secomatic Blue”. In particular, that document discloses a lithographic ink composition, wherein said ink composition cures when exposed to an excimer light source tuned to a wavelength of 300 nm or longer. The photoinitiator combinations disclosed are principally aminoalkyl phenones, phosphine oxides and some benzophenone derivatives. A preferred embodiment includes a composition with 4-6% 4-benzoyl-4′-methyl diphenyl sulphide with 2-4% of the amino benzoate synergist ethyl-4-(dimethylamino) benzoate cured using a 308 nm light source. Similarly, WO 2009/008226 (Toyo ink manufacturing company) entitled “Ink curable with actinic energy ray and printed matter” describes an ink suitable for use with an LED curing lamp emitting in the wavelength range 350 to 420 nm. Also described is the composition containing a photo cleavage type photoinitiator such as an aminoalkyl phenone and/or a phosphine oxide photoinitiator, and a hydrogen abstraction photoinitiator which is a dialkylamino benzophenone. The formulation also optionally contains a tertiary amine compound. WO2004/056581 (Inca Digital Printer Ltd and Sericol Ltd. Curing) describes a method of curing of a UV curable inkjet ink based on a monochromatic (typically LED) light source in an inerted environment. Also described is a series of photoinitiator possibilities for curing these inkjet inks in a nitrogen inerted environment which include aminoalkyl phenones such as Irgacure 369, and a photo sensitizer such as a thioxanthone. The ink formula D in Example 2 uses the combination of 8% Irgacure 369 and 2% isopropyl thioxanthone. JP 262068752 (Seiko Epson Corporation) describes an ink composition comprising N-vinyl polymerizable compounds and a photo polymerization initiator of two or more kinds, selected from bisacylphosphine oxides, monoacylphosphine oxides and aminoalkyl phenones. JP 28280460 (Sakata Corporation) describes a cationically curable UV inkjet ink containing a cationic polymerization initiator and a sensitizer that develops a sensitizing function by light of a wavelength of around 400 nm. JP 29035650 (Sakata Corporation) describes the use of a photopolymerizable compound, 5-30 wt % of an acrylated amine compound having two photopolymerizable functional groups and two amino groups in the molecule and 5-20 wt % of a compound exhibiting initiator function by light having a wavelength of 300-450 nm. WO 2007/017644 (Sun Chemical B.V.) describes a cationic inkjet ink suitable for curing using a UV LED light source comprising an iodonium salt photoinitiator and thioxanthone sensitizer. The article “Sensitization of photoinitiators by triplets sensitizers”; K Dietliker et al. Radtech 1987 conference, Florence, page 3-37 describes in detail the potential to create curing radicals from aminoalkyl phenone type photoinitiators using light of a wavelength they do not absorb by utilizing a triplet energy transfer mechanism from a thioxanthone compound which does absorb light in the irradiated spectral region. In particular, this article describes the effect for combinations of Irgacure 369 or Irgacure 907 in combination with thioxanthone derivatives such as isopropyl thioxanthone. These “sensitizer blends” are now well known to those skilled in the art.
JP 2010-59334 (Toyo Ink Manufacturing Company Limited), published 18 Mar. 2010 and which does not constitute part of the state of the art with respect to the present invention, describes an ink curable using a UV LED light source that includes: (A) a thioxanthone; (B) an α-aminoalkyl phenone; (C) an ethylenically unsaturated monomer; and (D) a tertiary amine synergist (paragraph [0010]). Suitable tertiary amine synergists are listed in paragraph [0029] and include N,N-dimethylamino p-benzoic acid esters.
Of the materials which are available, the generally accepted view by those skilled in the art is that monoacylphosphine oxide or bisacylphosphine oxide photoinitiators are the most effective for use with UV LED light sources, with dialkylamino benzophenones (particularly N,N′-diethylamino benzophenone), thioxanthones and aminoalkyl phenones being other valuable alternatives. However, the use of high levels of phosphine oxides initiators necessitates several unwelcome health and safety labeling categories such as R43 (sensitizing) and R 50/53 damaging to the environment. In addition, dialkylamino benzophenone derivatives such as ethyl Michler's ketone are commonly used in Japan but are unacceptable from a commercial standpoint for use in Europe because of their structural association with Michler's ketone, a known human carcinogen.
Sufficiently rapid curing of inks and coating, coupled with adequate levels of cure at the surface, remains difficult to achieve with UV LED light sources. Even relatively small improvements in cure speed and/or level of surface curing make a significant difference to the commercial viability of inks and coatings that are curable using UV LED light sources. Thus, there remains a need for improved ink and coating formulations that are suitable for curing using UV LED light sources.